1. Field of Invention
This invention relates generally to the read/write heads in optical information storage and retrieval systems and, more particularly, to the read/write head optical and sensor component configuration which generates the data, tracking, and focusing signals as a result of processing the resulting radiation beam, the radiation beam resulting from interaction with the radiation beam with a data track or grooves in the storage medium.
2. Description of the Related Art
Referring to FIG. 1, one configuration for an optical information storage and retrieval system, according to the related art, is shown. A radiation source 11, typically a laser diode, provides a radiation beam which is collimated by collimating lens 12. The collimated radiation beam is transmitted through polarization beam splitter 13 and applied to quarter wave plate 14. The polarization beam splitter 13 provides a linear polarization for the radiation beam and the quarter wave plate 14 provides a circular polarization to the radiation beam. The circularly polarized radiation beam transmitted by the quarter wave plate 14 is focused by objective lens 15 on the information storage surface 10A of the storage medium 10. The storage medium 10 is typically a disk with a surface which interacts with the circularly polarized radiation beam. The interaction with the storage medium surface 10A causes the radiation beam to be reflected and diffracted therefrom. The resulting information beam is collimated by objective lens 15 and the collimated resulting radiation beam transmitted through the quarter wave plate 14. The quarter wave B plate restores the linear polarization of the radiation beam. However, a component polarization perpendicular to the original polarization of the radiation beam will typically be present as a result of the interaction of the radiation beam with the storage surface. When the radiation beam is applied to the polarization beam splitter 13, the perpendicular component resulting from the interaction of the radiation beam with the storage surface 10A will be reflected by the beam splitter 13 while the polarization component parallel to the polarization component generated as a result of transmission through the polarization beam splitter 13 will pass through the polarization beam splitter. The reflected radiation beam is applied to sensor focusing lens 16. The lens 16 converges the reflected radiation beam on sensor array 5. The reflected beam has imposed thereon modulation that can be processed to provide the information (or data) which is stored on the disk. In addition, the reflected beam can be processed in such a manner as to provide tracking and focusing signals which can be used to activate apparatus which controls the position of the focused radiation beam on the storage surface 10A (i.e., the tracking in one dimension) and which controls the distance of the objective lens 15 from the storage surface 10A, (i.e., the focusing of the radiation beam on the storage surface). In this type of optical information storage and retrieval system, the quarter wave plate imparts, to the radiation beam illuminating the storage surface 10A, a circular polarization. After interaction with the storage surface, the quarter wave plate restores the linear polarization, however, the linear polarization will be rotated from the plane of polarization originally established by the polarization beam splitter 13. The rotated linearly polarized radiation component of the radiation resulting from interaction with the storage surface 10A is reflected by the beam splitter 13 and applied to sensor array 5.
Referring to FIG. 2, an example of the use of the processing of the radiation beam to provide tracking and focusing signals, according to the related art, is shown. This example is taken from European Patent Application 0,177,108 A1, issued in the name of A. Smid, P. F. Grave, and H 't Lam, entitled "Opto-Electronic Focussing-Error Detection System", and filed on Feb. 10, 1985. In this Figure, the path of the resulting radiation beam, the radiation beam which has already interacted with data track 21, is shown. (The quarter wave plate 14 and the beam splitter 13 have been omitted to emphasize certain important aspects of the configuration.) The data track or groove 21 is the path on the storage surface (10A of FIG. 1) along which the radiation beam will move in accessing or storing the information. A dual prism 25 is shown interposed between the objective lens 15 and the sensor focusing lens 16. The dual prism divides the resulting radiation beam into two radiation components. The two radiation components are essentially 1.), the radiation component reflected and radiation component diffracted from a first side of the storage medium and 2.) the radiation reflected and radiation diffracted by a second side of the storage medium, the two sides being separated by a median line of the data track. The first radiation beam component is focused on dual sensor elements A and B of the sensor array 5, while the second radiation beam component is focused on dual sensor elements C and D of sensor array 5. As will be known to those skilled in the art of processing resulting radiation beams, the data signal DS, the focusing signal FS, and the tracking signal TS are given respectively by: EQU DS=A+B+C+D 1.) EQU FS=(A+D)-(B+C) 2.) EQU TS=(A+B)-(C+D) 3.)
where A, B, C, and D of the Equations 1-3 represent the voltages developed by the equivalently designated sensor element when radiation is applied thereto. The data signal DS is the sum of voltages developed by all of the sensors elements. The focusing signal FS is the difference between the sum of the voltages resulting from the radiation applied to a first pair of diagonal sensor elements, i.e., A and D, and the sum of voltages resulting from the complementary diagonal pair of sensors, i.e., B and C. When the absolute value of the focusing signal FS is minimized, the radius of the radiation beam on the storage surface 10A will be minimized, i.e., the radiation beam will be focused on the storage surface. The tracking signal TS is minimized when the radiation reflected and diffracted from one side the center of the data track and from the other side below the center of the data track are equal. In order to understand how the tracking signal is derived, the role of the diffraction of the radiation beam must be understood.
Referring to FIG. 3A, the objective lens 15 is shown focusing the circularly polarized radiation beam on the storage surface 10A of storage medium 10. The storage surface 10A is shown as having a multiplicity of grooves, or equivalently, a multiplicity of data tracks 10B fabricated therein. The grooves 10B have dimensions relative to the wavelength of the radiation beam whereby diffraction patterns are formed. The data tracks 10B can be replaced with series of raised regions which are not connected, can be replaced with regions of appropriate dimension and refractive index, or any other structure which provides diffraction patterns in response to an impinging radiation beam without departing from the scope of the present invention. Referring to FIG. 3B, the resulting radiation beam after interaction with the storage surface is shown. The resulting radiation beam includes a zeroth order (reflected) component and a positive and a negative diffracted component. As will be clear, higher order diffraction components can be present, however, the present invention can be understood without further consideration of these components. The impinging radiation beam is shown as being off center and therefore closer to one edge of the data track or groove which is currently being tracked. This asymmetric positioning causes a wavefront phase shift in the diffracted orders and, consequently, an asymmetric interference between each of the diffracted components and the undiffracted (i.e., reflected or zeroth order radiation component). As a consequence, constructive interference occurs in one region, e.g., the region of overlap between the reflected radiation component and the + diffracted radiation component, while destructive interference occurs between the reflected radiation component and the -1 diffracted component. The magnitude of the resulting signal depends on the amount of shift of the impinging beam relative to the center of the data track or groove. In FIG. 3C, the difference between the intensities of the regions of interference is illustrated by region 32 (wherein the undiffracted radiation component and the +1 first order interference component interfere) and region 34 (wherein the undiffracted radiation beam component and the -1 first order diffracted radiation beam interfere). The polarity depends on whether the tracking of the radiation beam occurs for the data tracks (or grooves) or for the lands, i.e., the regions between the data tracks or grooves. Note that in the preferred embodiment, the two first order diffraction components are contiguous with the optic axis of the radiation beam. As a consequence, the two first order diffraction components will be superimposed on and will interfere with the reflected radiation beam. Referring once again to FIG. 2, the projection of the first order diffraction patterns 29A and 29B are shown on objective lens 15 and on dual prism 25. The difference in intensities of the resulting radiation components separated by dual prism 25 is determined by the relative intensities of the radiation components resulting from the interference between the undiffracted (reflected) radiation component and the first order diffraction components. It will be clear that the groove can be replaced by a diffracted and undiffracted radiation components resulting from applying a radiation beam to a data track without an associated groove, the data track implemented to provide the requisite diffracted and undiffracted radiation components.
The configuration for providing tracking signals and focusing signals, as disclosed by the Smid, suffers from a significant amount of optical cross-talk, generally originating from ever-present wavefront aberrations and the diffraction radiation components. Referring to FIG. 4, presence of optical cross-talk between the tracking signal and the focusing signal is illustrated. The presence of this optical cross-talk becomes particularly important in high performance signals such as are required in the information storage and retrieval systems.
In U.S. Patent Application Ser. No. 07/998,179 filed on Dec. 29, 1992, now abandoned in the name of David B. Kay, entitled APPARATUS AND METHOD FOR A DUAL HALF APERTURE FOCUS SENSOR, and assigned to the assignee of the present invention, a read/write head configuration is disclosed which minimizes the cross-talk between the tracking signal and the focusing signal. Referring to FIG. 5, the configuration of optical and electrical components which provide data, tracking, and focusing signals while reducing the optical cross-talk, according to the Kay application, is shown. As in FIG. 2, the apparatus interacts with the resulting radiation beam, i.e., the radiation beam which has interacted with the storage medium 10. Other components, such as the quarter wave plate shown in FIG. 1, have been omitted for clarity. The resulting radiation beam is recollimated by objective lens 15. The first order diffraction components 29A and 29B are shown in FIG. 5 by shadowing on objective lens 15. As will be clear, the reflected radiation component is also present and collimated by the objective lens 15. The collimated radiation beam is applied to beam splitter 52 where a portion of the collimated radiation beam is reflected and applied to dual element sensor 51, the dual element sensor having sensor elements E and F. Each of the sensor elements E and F have applied thereto a portion of the collimated and reflected radiation beam which includes only one of the two first order diffraction components. The remainder of the collimated radiation beam transmitted by beam splitter 52 is applied to dual prism 55. The dual prism 55 divides the applied radiation component into two focusing radiation beam components. Comparing dual prism 55 which dual prism 25 of FIG. 2, dual prism 55 is rotated 90.degree. with respect to a projection of the data track 21 on the prism. Therefore, the focusing radiation components include portions of both first order diffraction components as illustrated by the shadowing shown on the dual prism 55. Sensor focusing lens 16 focuses the radiation component from each prism element of the dual element prism 55 on one of the dual element sensors 5. The first dual element sensor has elements A and B associated therewith, while the second dual element sensor has sensor elements C and D associated therewith. The disclosed configuration, as shown by inspection of FIG. 5, includes in a separate path for the tracking and for the focusing signals. The separate paths diminish the intensity of the radiation beam at the detectors and require additional space and components. In typical optical storage systems, having a read/write head, the space on the read write/head is typically limited.
In U.S. Pat. No. 4,665,310 entitled "Apparatus For Optically Scanning An Information Plane Wherein A Diffraction Grating Splits The Beam Into Two Sub-Beams" and issued on May 12, 1987 in the name of Heemskert, a dual diffraction grating has been used in place of the dual prism to separate the radiation beam into two components. The separated components are thereafter used to provide the focusing and tracking signals. However, the cross-talk (coupling) between the tracking signal and the focusing signal has been found to limit high performance operation of an optical read/write head.
A need has therefore been felt for apparatus and an associated method for an improved optical read/write head in which diffraction gratings are used to process components of a radiation beam resulting from interaction with a storage medium. After processing, the radiation beam components are typically applied to radiation sensors and used for the generation of tracking signals, focusing signals, and the data (or information) signals. In this type of grating based read/write head in an optical information storage system, a need has been felt for a read/write head in which the cross-talk between the tracking and the focusing signals are minimized and in which a single return path is used for processing of the resulting radiation, i.e., the radiation which has interacted with a data track in the storage medium.